1. Field of the Invention
This invention relates to acousto-optic devices, and more particularly to acousto-optic devices having acoustic termination structures amenable to batch processing.
2. Description of Prior Art
All acousto-optic devices require some form of acoustic termination. Without termination, the acoustic energy is reflected back toward the transducer, creating interference with the acoustic wave being generated by the same transducer. The worst case is for a polished surface parallel to the transducer surface. In this case, almost all the acoustic wave is reflected directly back to the transducer.
FIG. 1 illustrates an acousto-optic device, interference, and acoustic termination. An optical beam (12) enters one window (14) and travels through the substrate (i.e. the crystal used to fabricate the device), exiting through a second window (16). Both windows are coated with an optical anti-reflection coating to maximize insertion and minimize reflection in the substrate. In most standard modulators, the acoustic wave (18) travels orthogonal to the optical beam (12). The acoustic wave generated by transducer (20) at bond face (22) interacts with the optical wave, deflecting it (20). For the device to function optimally, the acoustic wave must be attenuated and prevented from interacting multiple times with the optical wave, or returning to the transducer to interfere with the acoustic waves being generated there.
There are four ways to prevent an acoustic wave from reflecting off the backside and returning to the bond face: (a) absorb the wave on the backside surface; (b) transmit the wave through the backside surface; (c) scatter the wave; and (d) reflect the wave in another direction. These techniques can be explained by comparison with their more commonly understood optical counterparts. When an optical wave strikes a black matte surface, very little light is reflected off. With the acoustic equivalent of a black matte surface, there is a very low reflection of the acoustic wave. We have tried dense glues, platinum paint, and tungsten imbedded epoxy for acoustic termination. This technique has the advantage of a liquid staring material that is easily applied without any grinding or mounting step. The disadvantage is out-gassing due to the liquid form. While some improvement was measured, the results were not consistent. Acoustic transmission through the backside is equivalent to an optical antireflection coating. The difficulty is in developing good coatings and very accurate deposition and measurement techniques.
All surfaces, even polished, will scatter an acoustic wave somewhat. A rougher surface will scatter more. Ideally, a surface could be made arbitrarily rough to produce the required termination level. However, the rougher the surface, the more susceptible the substrate is to cracking and chipping, reducing device yields. Rougher surfaces are also more difficult to clean, affecting yields.
Backside angles are the most direct way to significantly reduce the acoustic wave reflection. Single or compound angles may be used for acoustic termination. A single angle can either have the angle between the windows (in the direction of optical propagation) or perpendicular to the direction of optical propagation. A compound angle has an angle both in and perpendicular to the direction of optical propagation. See FIG. 2. We have found a compound angle to be more effective. Some standard modulator angles are 7°×26°, and 7°×40°. For these angles, termination levels of 40-60 dB are commonly measured. We have found that 38 dB is a minimum termination level for good device performance. Backside angles are commonly combined with some backside roughening. Current devices employ a ground surface at a compound angle on the backside to both reflect away and scatter the incident acoustic wave. The compound angle is very effective at removing the acoustic interference; however, the angle is ground as one of the final fabrication steps, and must be done individually for each device. This is a major disadvantage when processing large numbers of devices, since it requires separate handling and mounting of each device.
Angled devices are manufactured by slicing a crystal of the desired size from its cell, then wax mounting the crystal on the transducer surface on a compound angle backside grinding fixture. The electrode and/or transducer can be damaged during this step, and each device must be carefully mounted one after another onto the backside grinding fixture. If the amount of wax is not adequate, the devices can be pulled off during the grinding and damaged. After grinding, the fixture is heated again to demount the devices, and the devices are soaked for many hours in a solvent to remove the wax. While this process is adequate for lower volume products, it is a bottle neck for larger volumes.